Ever go to the Louvre and wonder how the hell Caravaggio made his faces so crazily lifelike? A controversial book suggests that the Old Masters were quietly getting some technological help on the side. Artist David Hockney has a new book — Secret Knowledge — that claims da Vinci and his contemporaries were using camera obscuras and mirrors to project their real-life subjects onto the canvas, and then simply tracing the images. As CBS reports:

Hockney says it started in Bruges, Belgium, one of Europe’s great 15th century commercial centers, where that optical look, a photographic look, first appeared in the works of Flemish masters like Jan van Eyck.

“[He was] a painter who knew about optical projections and had looked at them,” says Hockney. “One thing the mirror projections do is project surfaces quite amazingly, especially shiny surfaces. And there’s lots of shiny surfaces.”

As Hockney points out, plenty of artists like Leonardo da Vinci were keenly aware of the camera obscura — hell, they practically invented and refined the device. And the “smoking gun,” Hockney argues, is that as soon as Old Masters art became hyperrealistic, there was a profusion of paintings of left-handed people. That would seem to support the idea that the artists were tracing right-handed figures that had been reversed in a camera obscura.

Art critics argue, quite rightly, that there is no written evidence proving Hockney’s theory; no artist of the period has ever discussed using optics and tracing. But there may be a reason for that:

Even today, he says, the artists wouldn’t tell: “They’re very secretive. Remember, they’re competing in business as well.”

It was also the time of the Inquisition, when mirrors and lenses were associated with witchcraft.

“When Caravaggio is painting in Rome, around the corner in the square, they’re burning Claudio Bruni for looking through lenses,” says Hockney.

Personally, I doubt the tracing thesis, for one simple reason: I simply don’t think the Mona Lisa looks particularly lifelike at all. Truth be told, she creeps the hell out of me. That weird pointy chin. Yeeee.